One of the inherent problems in the burning of wood products is that for efficient burning, the moisture content should be reduced as low as possible, preferably below 10%. The major potential source of wood fuel is pulp chips, although considerable amounts of shavings and sawdust are available from the lumber industry. This material may contain as much as 50% water, sometimes more, most of which should be removed before burning.
Conventionally, more finely divided materials may be dried in a rotary drum dryer, where the particles are suspended in hot air as they pass through the dryer. This is feasible with small or thin particles, because the time of retention in the dryer is very short. However, as the particles become larger, retention time must be greatly increased because of the low thermal conductivity of the wood. In drying a piece of wood, it is necessary to provide all the heat for evaporation of the water by transfer of the heat from the surface of the particles to the center of the particles. This requires that the whole particle must be heated beyond vaporizing temperature, as sufficient time must be allowed for the transfer of the heat to the center of the particle and for the diffusion of the water vapor to the surface of the particle. Thus, as the particle becomes larger, the retention time for drying at a given temperature and air flow will be greatly increased. For example, at 300.degree. F. with an air velocity of 500 feet per minute, the drying time of a particle 0.010 inches thick may be from 1 to 2 minutes, while a pulp chip having a thickness of 1/2 inch may require more than thirty minutes. Because of this extended drying time, the holding capacity of the dryer for chips must be far greater than that of the suspension type dryer processing fine materials, but giving the same rate of evaporation per hour.
A further complication in the drying of chips is in the presence of a small percentage of fine particles which could ordinarily be dried in suspension. While the suspension type dryer provides for the separation of suspended particles in the air at the discharge of the dryer, the major portion of chips supplied consists of particles so heavy as not to be readily air borne and therefore cannot be dried in the stream of air which would otherwise lift the fine particles. The objective of this invention is to provide means for drying a mixture of chips and fine particles without suspending fine particles in the discharge air steam and to provide adequate retention time to complete the drying of relatively heavy particles such as chips.
A principal application of the subject dryer is in the processing of fuel chips as they are fed to a wood burning device using the heat from the combustion gases emerging from the burner as a source of heat for drying the incoming fuel. Thus, a major objective is to maintain drying efficiency at a widely varying rate of input to balance with the burner firing rate.